While alien crosstalk is a limiting factor in 2.5GBASE-T and 5GBASE-T applications, Category 5e and Category 6 cabling do not include alien crosstalk specifications. Consequently, TIA TSB-5021 and ISO/IEC TR 11801-9904 performance specifications were introduced to address the evaluation of existing Category 5e and 6 cabling for support of these applications. These documents characterize extended frequency for Category 5e performance above 100 MHz (to support 5GBASE-T) and include qualification specifications, including internal cabling parameters (insertion loss, return loss, near-end crosstalk, etc.) and alien crosstalk.
Alien crosstalk performance for support of 2.5/5GBASE-T applications is determined using a formulation called ALSNR, Alien Limited Signal to Noise Ratio. ALSNR combines calculations for insertion loss, alien near-end crosstalk (NEXT) and alien far-end crosstalk (FEXT).
It’s well known that longer, tighter cable bundles exhibit more alien crosstalk, and assessment can be made to determine which channels may be at risk. This assessment can be done by reviewing cabling documentation, previous test results and visual inspections.
This table from the NBASE-T Alliance is a good guide for determining risk—the longer the length of bundled cabling, the higher the ALSNR risk. This is where good documentation comes in handy—test results from the time the cabling plant was deployed can be sorted by length to determine which links are vulnerable. Visual inspection will let you know if these links are tightly bundled.
Source Fluke Networks